In a conventional crosswind landing approach, the pilot places the aircraft in a "crabbed" configuration wherein the aircraft is headed to the left or right of the runway and into the wind in order to maintain a track along the runway centerline. At a selected altitude above the runway, the pilot adds sufficient rudder (decrab) to swing the longitudinal axis of the aircraft into alignment with the runway in preparation for landing touchdown and rollout. At the same time, the pilot banks the aircraft into the wind sufficiently to maintain the desired ground track in alignment with the runway. This decrab maneuver is one of the more demanding control tasks for pilots of current generation aircraft, and it requires carefully coordinated "crossed controls" i.e., opposing control wheel (or control stick) and rudder pedal inputs. While crosswind landings in a crab can be accomplished in light to moderate crosswinds with certain aircraft, a partial or full decrab maneuver is generally needed for landings in more severe crosswind conditions.
For autopilot controlled landing approaches (autoland approaches) where the pilot is not in the primary control loop, a number of automatic decrab control laws have been developed. For example, U.S. Pat. No. 2,987,276 to Osder discloses an autoland decrab system which uses instrument landing system (localizer) information to decrab the aircraft and lower the windward wing to maintain the desired ground track in alignment with the runway centerline until landing touchdown.
A disadvantage of these autoland decrab systems is that in the event the autoland system malfunctions, the pilot is required to disengage the autopilot landing system and resume manual flight control. If the pilot elects to continue the landing, he is faced with having to manually perform the demanding decrab and flare control tasks.
Another patent discusses a limited pilot aid method for pilot-in-the-loop decrab control. In U.S Pat. No. 3,761,691 to Schultz et al there is provided a "flight director" type of roll steering command guidance which instructs the pilot as to how he should move his wheel/stick in order to hold track.
Another asymmetric flight condition of interest is an engine out condition on a multi-engine aircraft with noncenterline thrust. During this condition the aircraft will yaw to the right or left due to the asymmetric thrust and drag associated with the noncenterline engine failure. The yawing moment can be particularly severe and difficult to control for an engine failure in a high thrust flight condition such as takeoff climb or go-around. In order to reduce aircraft sideslip angle consequent to the engine failure, opposite rudder may be applied by the pilot. While the pilot may apply either opposite rudder or no rudder, he must apply a wheel/stick input to bank the aircraft with the good engine down in order to maintain aircraft flight along the original track angle.